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Lecture

WEB Influence of Dislocations Introduced by Plastic Deformation on Thermal Conductivity of Strontium Titanate

Wednesday (01.01.2020)
00:15 - 00:30
Part of:


Good thermoelectrics (TEs) have low thermal conductivity, e.g. due to their weak interatomic bonding while further reduction of the thermal conductivity is achieved by additional phonon scattering at lattice defects such as dopants [1]. The impact of dislocations on thermal conductivity is well known for cryogenic temperatures but has been suggested to also occur at room temperature. Recently, a decrease of thermal conductivity with the logarithm of dislocation density between dislocation densities of 1011 m-2 to 1014 m-2 was shown for GaN at 300 K [2]. As plastic deformation is an industrially scalable process to introduce dislocations, its feasibility to tune thermal conductivity has been investigated in detail for both, single crystals and polycrystals.

For strontium titanate (SrTiO3), the relatively high thermal conductivity limits its competitiveness as a TE [3]. On the other hand it displays outstanding plastic behavior, which is well known for its single crystals [4], making it a suitable model material. The feasibility of plastic deformation to reduce thermal conductivity in TEs was tested by deforming [110]-oriented, undoped SrTiO3 single crystals at 1050 °C producing well-ordered {100}<100> type dislocations. Furthermore, undoped SrTiO3 polycrystals were deformed by 1.7% at 1150 °C. TEM images give evidence for dislocation densities of 10^12 m^-2 and 2×10^12 m^-2, respectively.

Thermal conductivity was determined using Laser Flash Analysis between room temperature and 325 °C. With none of the dislocation densities, noteworthy reduction of thermal conductivity was achieved. Results were corroborated for <110>{110} type dislocations in slip bands, Nb-doped and Fe-doped single crystals. Complementary calculations of thermal conductivity using the Debye-Callaway model predict a significant reduction of thermal conductivity only above a dislocation density of 1015 m-2. Hence, modest plastic deformation appears to yield insufficient dislocation densities to further reduce thermal conductivity of SrTiO3. Due to the need for a substantial further increase of dislocation density beyond 10^15 m^-2, significantly increased degrees of deformation, far beyond the demonstrated 2 %, are required in order to consider plastic deformation as a suitable approach to reduce thermal conductivity.

 

Speaker:
Melanie Johanning
Technische Universität Darmstadt
Additional Authors:
  • Lukas Porz
    Technichal University of Darmstadt
  • Jinfeng Dong
    Tsinghua University Beijing
  • Prof. Dr. Jing-Feng Li
    Tsinghua University Beijing
  • Prof. Dr. Atsutomo Nakamura
    Nagoya University
  • Prof. Dr. Jürgen Rödel
    Technical University of Darmstadt